His new film, World's Greatest Dad, is a glorious return to form. But a mournful Robin Williams would rather talk about his battle with drugs and alcohol – and recovering from heart surgery

Decca AitkenheadMonday 20 September 2010 08.00 BST

In the normal order of things, an interview with a Hollywood actor observes the form of a transaction. The actor wants to promote their film, and ideally talk about little else – least of all anything of a personal nature. The newspaper is mildly interested in the new film, but hopes they can be tempted to talk about other matters – best of all their private life. Sometimes the agreement is explicit, but most of the time it is mutually understood, and so the interview tends to proceed rather like a polite dance, with each party manoeuvring in its own interests. On this occasion, however, the convention appears to have been turned on its head.